Desk-top sized printing systems are now in common use for preparing small quantities (a few per day) of ID cards on an as-need basis. Such ID cards typically include a photographic image of a person being issued with the card in addition to the name of the person and possibly other information concerning the person.
These ID-card systems include dye-sublimation systems and laser engraving systems. Dye sublimation printing has an advantage that a color photograph can be printed. The photograph information however is fragile, which requires that a laminated transparent cover must be provided for their protection. In a laser engraving system an ID card is produced by burning a photograph and information into a light colored (usually white) plastic card using a focused laser beam. This provides a much more durable image than a dye-sublimation printed image. The laser-engraved image usually does not require a laminated cover for protection. Further, the laser engraving systems do not require consumables such a printing-ink or dye cartridges, toner, or the like.
A type of laser typically used in such engraving systems is a Q-switched, pulsed near infra-red (NIR) laser including a solid-state gain-medium such as neodymium-doped YAG (Nd:YAG). This type of laser typically delivers a beam of pulses having a duration between about 10 and 100 nanoseconds (ns) at a pulse-repetition frequency (PRF) between about 10 kilohertz (kHz) and 100 kHz. The peak power of the laser is usually required to be about one kilowatt (kW). The beam is focused on the card being printed, and is typically scanned by a two-axis scanning galvanometer, in vector fashion, over the card, in order to build up a pixellated image. The pixel-size in an image is determined by the size of the focal spot of the focused beam and is typically between about 30 and 100 micrometers (μm) in diameter.
A gray-scale image is produced by varying the laser flux in each pixel. In response to a laser flux, black grains grow within the pixel spot-size. The number of grains increases with increasing laser flux until the entire area of the spot is black. Gray scale is controlled by controlling the flux to control the area of the focal spot occupied by the black grains. This requires very fine control of the laser flux to provide consistent, reproducible results. While the degree of control required is not impossible to provide in a Q-switched pulsed laser, it adds considerably to the complexity and cost of the laser, and, accordingly, to the cost of engraving apparatus.